Offshore mobile drilling units are often used for offshore oil and gas drilling in water depths up to about 350 feet. Such drilling platforms can and have been converted to mobile offshore production platforms. With a typical surface well head, an outer layer of casing, which may be protected by a further outer layer or well conductor, is sunk to the required depth and terminates above the water surface. To avoid damage to the casing, it is essential to support it in some manner against the effects of wind and wave motion.
Such support has in the past been provided by various arrangements and constructions of supporting structures having one or more pylons embedded in the sea floor. As can be appreciated, the labor, material and time costs in creating such supports is not insubstantial, particularly when a mobile rig platform could be used.
Mobile drilling rigs generally are constructed in one of two basic platform deck configurations--slotted and cantilever. A slotted deck in a mobile drilling rig is one which includes an aft cut-out with the drill floor and rotary table mounted from the slot edges above the slot. A cantilever deck construction puts the drill floor and rotary table adjacent and above the aft end of the deck, supported from the deck by way of cantilever supports. Some mobile rigs, whether cantilever or slot, include tracks for moving the drill floor and rotary table into the precise position needed for drilling, to allow for the imprecision in precisely positioning the entire rig deck.
Current government regulations applicable to offshore production require that a casing string extending from the sea bed be supported in such a way that the string is capable of withstanding sea conditions that would be encountered in a hundred year storm without being dislodged from the sea bed. Consequently various types of supporting structures have been used to accomplish this result. Examples of such structures are shown in U.S. Pat. Nos. 4,932,811 to Folding, 5,012,875 to Casbarian, 4,907,675 to Cox, 4,842,446 to Carruba, and 4,958,960 to Turner et al. All of these involve one or more piles driven into the ocean floor from which lateral structures extend and provide support to the well conductor. Due to costs and mobility considerations, it would be desirable to have a well conductor support that does not require piles driven into the sea floor and yet is capable of providing sufficient support to the conductor so that it can withstand a hundred year storm. It would be particularly desirable for such a support to be capable of being integrated into a mobile platform such that the platform can be moved into position for production and the platform can then also serve as the sole casing string support structure.